Monday, January 21, 2019

(a) Current fluctuation in matter generates black-body radiation in far field and fluctuating evanescent field in near field. (b) Electromagnetic energy densities of evanescent field uev and propagating radiation upr produced by an oscillating single electric dipole, as a function of the distance normalized by wavelength.

A novel scanning microscope, which visualizes kinetics of charges by mapping ultrahigh frequency current fluctuation (15–30 THz), is described. This microscope, called the scanning noise microscope, scatters a fluctuating electromagnetic evanescent field on the sample surface with a sharp metal tip and detects the scattered field with an ultrahighly sensitive THz detector in a THz confocal microscope. This article describes the basic concept of the microscope, highlights the uniqueness and the general importance of the method, and demonstrates the powerfulness of the method by exemplifying experimental results made on (i) nanostructured metal layers in thermal equilibrium, (ii) narrow metal wires under non-uniform Joule heating, and (iii) operating GaAs nano-devices with non-local hot-electron energy dissipation in a highly non-equilibrium condition.

Three-dimensional microporous graphene (3DMG), possesses ultrahigh photon absorptivity and excellent photothermal conversion ability, and shows great potential in energy storage and photodetection, especially for the not well-explored terahertz (THz) frequency range. Here, we report on the characterization of THz-thermal-electrical conversion properties of 3DMG with different annealing treatments. We observe distinct behavior of bolometric and photothermoelectric responses varying with annealing temperature. Resistance-temperature characteristics and thermoelectric power measurements reveal that marked charge carrier reversal occurs in 3DMG as the annealing temperature changes between 600 and 800 °C, which can be well explained by Fermi-level tuning associated with oxygen functional group evolution. Benefiting from the large specific surface area of 3DMG, it has an extraordinary capability of reaching thermal equilibrium quickly and exhibits a fast photothermal conversion with a time constant of 23 ms. In addition, 3DMG can serve as an ideal absorber to improve the sensitivity of THz detectors and we demonstrate that the responsivity of a carbon nanotube device could be enhanced by 12 times through 3DMG. Our work provides new insight into the physical characteristics of carrier transport and THz-thermal-electrical conversion in 3DMG controlled by annealing temperature and opens an avenue for the development of highly efficient graphene-based THz devices.

For any laser, high power, good beam quality, and broad electrical frequency tuning range are deemed as the key assets to possess for various applications. Terahertz (THz) quantum cascade laser is no exception. Since its invention in 2001, extensive effort has been devoted to optimizing the laser performance, yet with little success in improving these major aspects on a laser chip simultaneously. Recently, A group led by Qing Hu in MIT has made a major leap with a phase locking approach and achieved these three key performance goals at once. Inspired by the chemistry of hybridization, they can phase lock multiple THz wire lasers by π coupling design. By properly adjusting spacing of adjacent laser element, the laser array can be designed to operate in a coherent symmetric supermode — all collectively radiating in a phase-locked scheme. The demonstrated device exhibits a good level of output power up to 90 mW at continuous-wave operation, and a tight beam pattern of 10° divergence, and a continuous electrical tuning of ~10 GHz at ~3.8 THz. Achieving all three performance metrics means less noise and higher resolution, for more reliable and cost-effective gas sensing, chemical detection, and medical imaging.

Sunday, January 20, 2019

In this article, terahertz imaging system based on a single‐channel 360 GHz transceiver front‐end and optical‐machine scanning platform, is studied. The transmitting antenna is irradiated by a wide beam of a small aperture antenna to reduce the “angular glint” effect caused by high frequency electromagnetic radiation. The receiving antenna is of a large aperture to ensure high resolution. Beam scanning is achieved through a fast conical scanning method using a total reflector to achieve fast real‐time imaging.

We explore the effect of quasistatic field generation in a nonstationary medium.
The nonstationarity is provided by time-dependent density of free carriers created by an
ultrashort laser pulse, a situation which is common in electro-optic schemes of THz generation.
The quasistatic fields can serve as indicators of ultrafast carrier dynamics and can be used for
particle acceleration, control of magnetic materials, streaking techniques, and spectroscopy.

The nonclassical properties of a microcavity confining two quantum wells irradiated by a coherent light source and interacting with a squeezed vacuum reservoir are investigated. By deriving analytical expressions of the intensity power spectrum, the squeezing spectrum, and the second-order correlation function of the output field, we show that the proposed scheme generates perfect squeezing. The obtained results are quite different from the single-quantum-well cavity system where, under the same conditions, squeezing does not exceed 50%, and the transmitted light is bunched. It turns out that the presence of indirect excitons in the cavity strongly modifies the quantum dynamics of the system.